The present invention generally relates to detection of ionizing radiation such as e.g. gamma and X-rays, and particularly to positron emission tomography (PET).
More specifically, the invention relates to a novel detector arrangement and a method, respectively, for highly temporally and spatially resolved detection of radiation, and to a positron emission tomography apparatus and a method, respectively, for construction of an image of an object of interest from linear projections of radiation as emitted from the object and subsequently detected at high speed and at accurately determined positions.
A PET detector or camera typically consists of a polygonal or circular ring of radiation detection sensors placed around a patient area. Radiation detection begins by injecting isotopes with short half-lives into a patient""s body placed within the patient area. The isotopes are absorbed by target areas within the body, and emit positrons. In the human body, the positrons annihilate with electrons. As a result thereof two essentially monoenergetic gamma rays are emitted simultaneously in opposite directions. In most cases the emitted gamma rays leave the body and strike the ring of radiation detectors.
The ring of detectors includes typically an inner ring of scintillation crystals and an outer ring of light detectors, e.g. photomultiplier tubes. The scintillation crystals respond to the incidence of gamma rays by emitting a flash of light (photon energy), so-called scintillation light, which is then converted into electronic signals by a corresponding adjacent photomultiplier tube. A computer, or similar, records the location of each light flash and then plots the source of radiation within the patient""s body by comparing flashes and looking for pairs of flashes that arise simultaneously and from the same positron-electron annihilation point. The recorded data is subsequently translated into a PET image. A PET monitor displays the concentration of isotopes in various colors indicating level of activity. The resulting PET image then indicates a view of neoplasms or tumors existing in the patient""s body.
Such detector arrangement is known to have a good energy resolution, but relatively bad spatial and temporal resolutions. Early PET detectors required a single photomultiplier tube to be coupled to each single scintillation crystal, while today, PET detectors allow a single photodetector to serve several crystals, see e.g. U.S. Pat. Nos. 4,864,138; 5,451,789; and 5,453,623. In such manner the spatial resolution is improved or the number of photodetectors needed may be reduced.
Nevertheless, relatively low spatial resolutions are still obtained by PET detectors employing scintillator-based photodetectors. Further improved spatial resolution requires the use of a large number of small photodetectors and a scintillator system, which generates light photons only in the scintillator segment in which the incident radiation was absorbed. The use of a larger number of photodetectors in a large array or to increase the resolution of the device results in very complex and expensive apparatus if at all realizable.
Further, in medical applications where it is desired to expose the patient to a minimum amount of ionizing, it is important that the detector device is sensitive to low levels of radiation while still being able to discriminate against background radiation. In some applications scintillator-based detectors may not possess sufficiently high sensitivities or signal-to-noise ratios.
Still further, the light detecting arrangement of the detector device is typically sensitive to direct irradiation by the incident radiation and in such instance measures have to be taken in order to prevent the incident radiation from reaching such arrangement.
Accordingly, it is an object of the present invention to provide a detector arrangement and method for detection of radiation, which simultaneously provide for very high temporal and spatial resolutions.
A further object of the invention is to provide a positron emission tomography (PET) apparatus and a method, respectively, for construction of an image of an object of interest from linear projections of radiation as emitted from the object and subsequently detected, which provide for a high speed and a very high spatial resolution of the image constructed.
Yet a further object of the invention is to provide such detector arrangement, PET apparatus, and methods, which provide for effective rejection of noise; thus exhibiting high signal-to-noise ratios.
Still a further object of the invention is to provide such detector arrangement, PET apparatus, and methods, which provide for spectrally resolved detection. In the case of PET detection an improved energy resolution can provide for a lower number of erroneously correlated signal pairs.
Yet a further object of the invention is to provide such detector arrangement, PET apparatus, and methods, which can operate at fairly high radiation fluxes and which thus provide for rapid examinations.
Still a further object of the invention is to provide such detector arrangement, PET apparatus, and methods, which are effective, accurate, reliable, easy to use, and of low cost.
These objects among others are, according to the present invention, attained by detector arrangements, PET apparatus, and methods, as claimed in the appended claims.
By providing a dual detector arrangement having a chamber filled with a scintillating and ionizable substance wherein light as well as electrons are created as a result of interactions between incident radiation and the substance, and in which the light and the electrons, respectively, are independently detected in a respective arrangement detector, i.e. in a light detector and in an electron avalanche detector, respectively, followed by correlation of the respective light and electrons which originate from the respective same incident radiation photon, detection may be performed which exhibit major advantages of both light detectors and electron avalanche detectors. Particularly, as light detectors are known to provide high energy resolution and electron avalanche detectors are known to provide high position and temporal resolutions, the inventive dual detector arrangement can provide the high energy resolution of the light detectors and simultaneously provide the high position and temporal resolutions of the electron avalanche detectors. Such detector arrangement is eminently adapted to be employed in positron emission tomography (PET) to reject otherwise erroneously matched signal pairs, by means of which images with reduced levels of noise can be produced.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.